Metallic pigments are known to provide electrochemical, electrical, thermal, barrier and other properties to compositions which are used for protecting materials such as metal from corrosion, maintaining electrical conductivity, shielding equipment from electromagnetic fields, resisting elevated temperatures, and providing protection from moisture. Silver, gold and other noble metal pigments are used for their electrical conductivity and thermal conductivity properties. Zinc and magnesium are used for their electrochemical properties. Aluminum is used for its thermal and chemical barrier properties. A major shortcoming of the noble metals is their strong cathodic potential. When used in products for electrical and thermal management, the noble metals can couple with anodic materials like aluminum alloys used for electrical equipment. Shortcomings of zinc pigments include low electrical conductivity and low thermal conductivity compared to the noble metals as well as their relatively poor resistance to chlorides and other corrosive materials. A shortcoming of magnesium pigments is their high relative anodic potential compared to other metals. In addition, magnesium forms a protective oxide spontaneously in air, rendering it less effective than zinc for the sacrificial protection of steel and other cathodic materials.
Coatings with aluminum powders have been used for over 100 years. These coatings are excellent barriers to the environment and provide good thermal stability and protection. Many bridges, tanks and other steel structures have been painted with aluminum-pigmented coatings over the years with much success. These coatings do not, however, provide galvanic or electrochemical protection of the metal surface on which they are coated, since the aluminum powder or flakes are covered with aluminum oxide which inhibits electrochemical action. These uses and shortcomings are well documented in “Aluminum Paint and Powder by J. D. Edwards and R. I. Wray, 3rd Edition, 1955. Magnesium, Zinc and Aluminum anodes are currently used in bulk form to protect metal from corrosion. To be effective, however, the anodes need to be in contact electrically with the object they are protecting when immersed in water or an electrolyte. There is extensive literature which describes the pros and cons of each anode material. Aluminum anodes are preferred, since they are inexpensive and provide the highest efficiency of the three metals.
Table 1 shows the electrical output and cost effectiveness of these three metals based on weight. With regard to recent spot prices for each metal and their relative cost effectiveness clearly aluminum is superior to zinc and magnesium and therefore preferred based on cost, weight and longevity. Table 1: Comparison of Electrical Properties of Magnesium, Zinc and Aluminum (from Reding, J. T. Newport, J. J.: The Influence of Alloying Elements on Aluminum Anodes in Sea Water. Materials Protection, Vol. 5. December 1966, pages 15-19).
TABLE 1Properties and Costs of Magnesium, Aluminum, and ZincMgAlZnTheoretical2.611.901.00Potential (volts)Electrical10001352372Output (amp hrs/lb)Metal cost (cents/lb)3524.514.5Cost of current at.035.018.039100% efficiency(cents/amp/amp-hr)
Before the 1970's aluminum anodes were not used for the same reasons stated herein for aluminum powders and flakes. The bulk material rapidly passivated, rendering the anode inactive and incapable of protecting the intended metal object. The development of activated aluminum alloys began in the mid-1960's. The intellectual property is documented in U.S. Pat. Nos. 3,379,636; 3, 281,239; 3,393,138 by Dow Chemical and U.S. Pat. No. 3,240,688 by Olin Mathesin. All of these alloys were unique in that for the first time bulk aluminum alloys were shown to remain active and galvanically protect metal. Unfortunately, none were commercially successful as they all suffered from low efficiencies making them less economical than zinc anodes. During the 1970's, Dow developed an aluminum anode alloy, which does not passivate and has very high efficiencies, approaching 90% of theoretical.
This invention relates to coated powder-pigments and to a process to coat aluminum alloy powder-pigments which are electrochemically active. An additional novel feature is the ability of the coating to transform electrically inactive standard aluminum particles, which are typically insulating due to the aluminum oxide coating which forms on them in the natural environment, to electrically active aluminum particles. It also includes the method to use these coated pigments in coating compositions intended to inhibit corrosion of metal substrates. Experiments show that an effective semi-conducting corrosion inhibiting coating can be produced on the aluminum alloy powder-pigments which inhibit self corrosion of the particles, but does not degrade the corrosion-inhibiting properties of the particles in the coating. Examples of the process to make these coatings and the method of using these corrosion-protected pigments in a corrosion-inhibiting coating are discussed herein.
It is therefore an object of this invention to incorporate electrochemically active powder-pigments into a binder to provide cathodic protection to metal substrates without the need of an external power supply.
It is another object to provide cathodic protection to metal substrates by coating the substrate with a sacrificial anode coating that keeps the electrochemical potential of the substrates negative to prevent its corrosion.